Automatic C++ doc generation

[JoffreyDorville]

Description

This PR adds support for automatic doc generation from C++ sources using doxygen and the python package breathe in addition to our existing sphinx configuration.

This allows to write functions docstrings for the C/C++ API directly in capi.h.
This will also help us write dedicated C++ docs more easily.

I translated a few functions from the docs folder to capi.h just to get a use case example. We can translate everything in a future PR. I imagine this is a perfect task for LLMs.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[JoffreyDorville]

I'll just update the developer documentation to add the new doxygen prerequisite.